In known art of internal combustion engines, a number of designs have been suggested for accomplishing valve drives, for inlet and/or exhaust valves, than can shift between different valve lift modes, at which the valve lift curves differ from each other. Shifts between said modes are usually accomplished through mechanical arrangements at the valve tappets.
There must be a high degree of certainty that shifts between the valve modes are not effected during certain critical stages of the valve lift operation. A reason for this is to avoid risks of damages occurring in the engine. Another reason is that it has to be possible to predict for each cylinder the cycle in which the mode shift is effected. If a mode shift is effected in a cycle different to the predicted cycle, there could be a lack of coordination between the valve lift and other parameters, such as the amount of air to be inducted, which in turn could result in a misfire. Therefore, it has to be made certain that mode shifts are effected in certain trigger windows between each valve lift process of the engine. These windows are usually relatively small, and at high engine speeds their time duration can be very small, increasing the risk, due to inaccuracy of the system, of a mode shift being effected at a critical stage of the valve train or in a cycle not predicted.